1. Field of the Invention
The present invention relates to an image forming optical system and an electronic image pickup apparatus equipped with the same.
2. Description of the Related Art
In recent years, image pickup apparatuses such as digital cameras have been replacing silver salt films and becoming widely used. Digital cameras are adapted to pickup an image of an object using a solid state image pickup element such as a CCD or CMOS. Taking lenses used in such image pickup apparatuses are desired to be zoom lenses (image forming lenses) having a high zoom ratio.
It is desired that such image taking lenses be satisfactorily corrected in terms of aberrations relevant to the monochromatic imaging performance (such as spherical aberration and coma). In addition, it is desired that correction of chromatic aberrations that are relevant to the resolution of images and color blur be achieved adequately.
On the other hand, the overall length of the lens (i.e. the entire optical length) is desired to be made shorter. However, the shorter the overall lens length is made in order to reduce the size of the entire optical system, the more aberrations, in particular chromatic aberrations, tend to be generated, and the lower the imaging performance tends to become. In particular, in the case of zoom lenses having a high zoom ratio and a long focal length at the telephoto end, a reduction of secondary spectrum is required in addition to first order achromatism when correcting chromatic aberrations.
As a method of reducing such chromatic aberrations, use of optical materials having extraordinary partial dispersion have been known (Japanese Patent Application Laid-Open No. 2007-163964, Japanese Patent Application Laid-Open No. 2006-349947, and Japanese Patent Application Laid-Open No. 2007-298555).
Furthermore, zoom lenses used in image pickup apparatuses are desired to have a certain zoom ratio, a wide angle of view at the wide angle end, high speed, and high performance. To improve the performance of a zoom lens, it is necessary to correct chromatic aberrations satisfactorily throughout the entire zoom range.